1. Field of the Invention
The invention relates to aqueous silicone emulsions which comprise high-viscosity polyorganosiloxanes and have a particularly low content of cyclic siloxanes, to processes for preparation thereof and to the use thereof.
2. Description of the Related Art
Silicones have a variety of uses. In order to facilitate application and metering, particularly in the case of viscous products, it is desirable for many applications that the organosilicon compounds are in dilute form. The use of organic solvents such as benzene or hydrochlorocarbons is possible for this purpose, but disadvantageous from an ecological and occupational health point of view. Therefore, use is usually effected in the form of aqueous emulsions or dispersions, typically in the form of oil-in-water emulsions (O/W emulsions) which can be diluted with water. The oil phase here is understood to mean the water-immiscible organosilicon compounds, optionally dissolved in organic solvents. For many uses, it is advantageous when the silicone has a high molecular weight and hence a high viscosity. A known way of arriving at emulsions comprising a high molecular weight silicone is the emulsion polymerization of low molecular weight, especially cyclic, organosiloxanes with arylalkylsulfonic acids (DE-A 14 95 512). In this context, through vigorous stirring or homogenization with a high-pressure homogenizer, exceptionally low particle sizes are achieved, these no longer being perceptible with an optical microscope. A disadvantage of this process is the fact that, because of the equilibrium character of this reaction, more than 10% volatile cyclic siloxanes are present based on siloxane, but are undesirable. It has therefore been proposed that these be distilled off subsequently (e.g. U.S. Pat. No. 4,600,436) or removed by a membrane process (EP-A 1 368 109). Both processes mean additional technical complexity and can impair the stability of the emulsion.
Alternatively, rather than cyclic siloxanes, linear oligomers having terminal silanol groups can be used. These oligomers, in the presence of emulsifiers, condensation catalysts and a very small amount of water, form a paste in which the polycondensation takes place. Subsequently, this paste is diluted to the desired concentration (EP 93 310 B2). In general, the proportions of cyclic volatile siloxanes are lower than in the case of emulsion polymerization of cyclic siloxanes. The proportion of these volatile siloxanes can be reduced, for example, by first producing an emulsion from the salt form of the anionic emulsifier/catalyst, then activating this emulsion by adding acid (EP-A 1 072 629). This ultimately increases the salt content in the emulsion, which is disadvantageous for stability. In the case of use of alkoxy-terminated siloxane oligomers, it is said that less cyclic siloxanes are likewise formed (JP-A2001288269). However, the production of these oligomers is more complex and hence more costly.
Specific emulsifiers based on taurocholates likewise make a contribution to the reduction in the amount of cyclic products which are formed in the emulsion condensation of siloxane oligomers (WO 2006102010). Here too, as the working examples clearly show, more than 1% octamethylcyclotetrasiloxane is formed.
There have also been suggestions of emulsifying dimethylpolysiloxanes, especially polysiloxanes terminated with trimethylsiloxy groups, having viscosities of up to 5,000,000 cSt, by mixing and heating them with 10-30% of a phosphoric partial ester based on siloxane until a clear solution has formed, which, after neutralization, is diluted with water (DE-A 27 30 923). However, this process has the disadvantage that the polydimethylsiloxane is usually depolymerized in the process, and so the emulsion obtained contains a low-viscosity siloxane and a high proportion of volatile cyclic siloxanes, e.g. octamethylcyclotetrasiloxane.
JP2002020490 proposes using, as emulsifiers, at least one two-substance combination of polyoxyethylene alkyl sulfates, polyoxyethylene alkyl phosphates and alkylsulfonates, or the corresponding acids, it being preferable that the acid is released in the emulsion only through addition of mineral acids such as sulfuric acid. Sole use of polyoxyethylene alkyl phosphates is said to lead only to low molecular weight polyorganosiloxanes, since the catalytic activity thereof is too low. Therefore, combinations with sulfates or sulfonates and activation of sulfuric acid are necessary. This ultimately leads in turn to more than 1% cyclic siloxane oligomers, unless the reaction time is extremely short, in which case, however, viscosities of >1,000,000 mm2/s are not achieved.
On the other hand, such emulsions are often produced practically in such a way that either several batches are produced batchwise and transferred into a maturing tank, or a continuous campaign is produced over a particular period in a maturing tank, where the reaction is stopped by neutralization after attainment of the desired viscosity. In this case, it is unavoidable that a considerable proportion of the emulsion resides in the tank for longer than required, as a result of which the proportion of cyclic oligomers exceeds the tolerable extent.